1. Field of the Invention
The present invention relates to a power supply for a negative voltage load that has good performance on power factor correction and effectively enhances the conversion efficiency of the power supply no matter whether AC power is normally supplied or not.
2. Description of the Related Art
With reference to FIG. 3, a conventional power supply for a negative voltage load (60) has a switch-mode power unit (50), a monitoring unit (54) and an auxiliary power circuit.
The switch-mode power unit (50) comprises a rectifying filtering circuit (51), a power factor correction circuit (52) and a DC to DC power circuit (53). The rectifying filtering circuit (51) is connected to an AC power through a switch circuit (511), converts AC power to DC power and outputs the DC power to the power factor correction circuit (52). The power factor correction circuit (52) adjusts current and voltage of the DC power to be in phase with each other. The DC to DC power circuit (53) then bucks the DC power and produces at least one stable DC voltage source to the negative voltage load (60).
The monitoring unit (54) is connected to a control terminal of the switch circuit (511) and an output terminal of the rectifying filtering circuit (51) to detect status of the AC power source and turn on or off the switch circuit (511) according to whether the AC power is stable.
The auxiliary power circuit has a first auxiliary power unit (55) and a second auxiliary power unit (56). The first auxiliary power unit (55) is connected to the output terminals of the rectifying filtering circuit (51) to convert DC power to a first DC operating power. The second auxiliary power unit (56) is connected to the first auxiliary power unit (55), receives and converts the first DC operating power and sends the converted first DC operating power to the monitoring unit (56). Furthermore, the second auxiliary power unit (56) is connected to output terminals of the DC to DC power circuit (53), connects to the negative voltage load (60) in parallel, converts the constant DC voltage source from the DC to DC power circuit (53) or a DC load voltage source from the negative voltage load to a second DC operating power and sends the second DC operating power to the monitoring unit (54).
The negative voltage load (60) is usually operated with −54 Volts and may be a battery pack or system bus. When the AC power source is normal and is supplied to the negative voltage source (60) after converting by the switch-mode power unit (50), the negative voltage load (60) will store negative power and the monitoring unit (54) in the mean time will detect whether the AC power source is maintained normal. If the AC power source is maintained normal, the negative voltage load (60) then continues to receive DC power. If the AC power source becomes abnormal, the monitoring unit (54) then turns the switch circuit (511) off to interrupt the AC power source.
When the switch-mode power unit (50) is supplied with regular AC power source, the operating power of the monitoring unit (54) is provided by the second auxiliary power unit (56), wherein the AC power source is converted to a second operating DC power source via the rectifying filtering circuit (51), the first auxiliary power unit (55) and the second auxiliary power unit (56) to be a suitable operating power for the monitoring unit (54). After the monitoring unit (54) controls the switch circuit (511) to be in a turn-off status, the first auxiliary power unit (55) no longer outputs the first DC operating power. In the meantime, the second auxiliary power unit (56) converts the negative power stored in the negative voltage load (60) to a second DC operating power to continue providing operating power to the monitoring unit (54) since the second auxiliary power unit (56) is connected to the negative voltage load in parallel. Therefore the monitoring unit (54) is maintained powered to continue detecting the status of the AC power source and is able to turn on the switch circuit (511) as soon as the AC power source normally supplies. The AC power source then is inputted to the switch-mode power unit (50) again to provide DC power to the negative voltage load (60).
However, foregoing power supply for a negative voltage load has disadvantages as follows:
1. When the AC power is normal, the operating power of the monitoring unit (54) is provided by both of the first and the second auxiliary power units (55,56), therefore the overall conversion efficiency of the power supply is low.
2. As for a negative voltage load with −54 volts and the monitoring unit (54) with +5 volts, the first and the second auxiliary power unit (55,56) need to be designed as isolation type power circuits and overall volume and complexity are increased.
3. Because the first auxiliary power unit (55) connected to the rectifying filtering circuit (51) receives DC power for conversion from the rectifying filtering circuit (51), the switch-mode power unit (50) must have a filtering capacitor connected between the rectifying filtering circuit (51) and the power factor correction circuit (52) to convert AC power to DC power. However, the filtering capacitor will deteriorates the power factor correction.
With further reference to FIG. 4, another conventional power supply for a negative voltage load similar to foregoing conventional power supply is disclosed for improving some of the aforementioned shortcomings. The difference is that the monitoring unit (54a) in FIG. 4 is connected to output terminals of the first auxiliary power unit (55a), the second auxiliary power unit (56a) is connected to the monitoring unit (55a) via the first auxiliary power unit (55a). Therefore, when the AC power is regular, the monitoring unit (54a) is powered only by the first auxiliary power unit (55a). When the AC power is irregular and is cut off by the monitoring unit (54a) via the switch circuit (511), the monitoring unit (54a) is powered by the second auxiliary power unit (56a) through the first auxiliary power unit (55a). Since AC power has much more time on providing regular power than being cut off, the power supply has better overall conversion efficiency than foregoing conventional one while the AC power is regular. However, when the AC power is cut off due to irregularity, the power supply still has bad conversion efficiency since the second auxiliary power unit (56a) has to go through the first auxiliary power unit (55a) to provide power to the monitoring unit (54a). Furthermore, the rectifying filtering circuit (51) still contains a filtering capacitor (CF) that causes negative effect on power factor correction of the power factor correction circuit (52).
To overcome the shortcomings, the present invention provides a power supply for a negative voltage load to mitigate or obviate the aforementioned problems.